In this kind of technical field, successors to the so-called third-generation mobile communication system are being studied by 3GPP, a standardization group for the wideband code division multiple access (W-CDMA) scheme. In particular, not only Long Term Evolution (LTE) but also further succeeding mobile communication schemes such as the IMT-Advanced scheme are being discussed, as successors of the W-CDMA scheme, the high speed downlink packet access (HSDPA) scheme, and the high speed uplink packet access (HSUPA) scheme and the like.
In a system such as LTE and the like, one or more resource blocks (RB) or resource units (RU) are allocated to a user apparatus (UE: User Equipment) both in downlink and uplink communications. Resource blocks are shared by multiple user apparatuses in the system. In LTE, the base station apparatus determines a user apparatus, among a plurality of user apparatuses, to which resource blocks are to be assigned every subframe which is 1 ms. The subframe may also be called a transmission time interval (TTI). The determination of assignment of radio resources is called scheduling. In the downlink, the base station apparatus transmits a shared channel using one or more resource blocks to a user apparatus selected in the scheduling. This shared channel is called a physical downlink shared channel (PDSCH). In the uplink, a user apparatus selected in the scheduling transmits a shared channel to the base station apparatus using one or more resource blocks. This shared channel is called a physical uplink shared channel (PUSCH).
In a communication system employing the shared channels, it is necessary to signal (or report) information indicating which user apparatus is assigned the shared channel in each subframe basically. A control channel used for the signaling is called a physical downlink control channel (PDCCH) or a downlink L1/L2 control channel. A downlink control signal may include, in addition to the PDCCH, a physical control format indicator channel (PCFICH) and a physical hybrid ARQ indicator channel (PHICH), and the like.
The PDCCH, for example, includes the following information:
Downlink scheduling information
Uplink scheduling grant, and
Transmission power control command bit
The downlink scheduling information may include information regarding a downlink shared channel, for example. More particularly, the downlink scheduling information may include downlink resource block assignment information, identification information of a user apparatus (UE-ID), the number of streams, information regarding precoding vectors, data sizes, modulation schemes, and information regarding hybrid automatic repeat request (HARQ).
The uplink scheduling grant may include information regarding an uplink shared channel, for example. More particularly, the uplink scheduling grant includes uplink resource assignment information, identification information of a user apparatus (UE-ID), data sizes, modulation schemes, uplink transmission power information, and information regarding a demodulation reference signal used in uplink MIMO, and the like.
The PCFICH is used to report the format of the PDCCH. More specifically, the PCFICH is used to report the number of OFDM symbols to which the PDCCH is mapped. In LTE, the number of OFDM symbols to which the PDCCH is mapped is one, two, or three. The PDCCH is mapped from a top OFDM symbol of the subframe in order.
The PHICH includes acknowledgement/non-acknowledgement information (ACK/NACK) indicating whether retransmission is necessary for the PUSCH transmitted via uplink.
In the uplink, the PUSCH is used to transmit user data (a normal data signal) and control information accompanying the user data. Also, separately from the PUSCH, a physical uplink control channel (PUCCH) is provided to transmit, for example, a downlink channel quality indicator (CQI) and acknowledgement information (ACK/NACK) for the PDCCH. The CQI is used, for example, for scheduling processing and adaptive modulation and channel coding scheme (AMCS) processing of the physical downlink shared channel. In the uplink, a random access channel (RACH) and signals indicating assignment requests for uplink and downlink radio resources may also be transmitted as necessary.
As mentioned above, in the system of the LTE scheme and the like, communication of a user apparatus is performed using one or more resource blocks. Which resource block is usable should be signaled (reported) for each subframe in principle. Radio resources are required also for performing signaling. Since the radio resource used for signaling becomes an overhead for commutation of the shared data channel (physical shared channel), it is preferable that the radio resource used for signaling is small from the viewpoint of efficient transmission of a shared data channel. Based on this viewpoint, in the LTE scheme, radio resources for retransmission of the Hybrid Automatic Repeat Request (HARQ) in the uplink are predetermined such that radio resources are shifted by a predetermined frequency at predetermined time intervals. That is, retransmission control in the uplink is performed using a predetermined frequency hopping pattern in the synchronization type ARQ scheme.
“Synchronization type” derives from the fact that timing for retransmission comes at predetermined intervals such as every 8 TTIs.
FIG. 1 shows a manner in which retransmission is performed using a predetermined frequency hopping pattern in the synchronization type ARQ scheme. A user apparatus transmits a signal using a radio resource shown as A1, and if retransmission is required, retransmission is performed after a round trip time (RTT). A radio resource used in the retransmission is shown as A2. It is checked whether retransmission is required in the period of RTT. Although not shown in the figure, the user apparatus checks whether retransmission is necessary by demodulating PHICH indicating the acknowledgement signal (ACK/NACK). If further retransmission is required after the user apparatus retransmits the signal using the radio resource of A2, the further retransmission is performed after RTT (A3). Each of the other users perform transmission and retransmission of signals in the same way (B1,B2,B3; C1,C2,C3).
A radio resource occupies one subframe (TTI) and a bandwidth (RB) of one or more resource blocks. Assignment of radio resources for each user is updated for each subframe, and transmission and retransmission of a signal are also performed for each subframe, in principle. However, the signal of one subframe does not necessarily provide proper reception quality at all times. For example, quality of a signal from a user apparatus residing in a cell edge tends to become lower than quality of a signal from a user apparatus near a base station. For addressing such concern, there is a technique called subframe bundling (also may be called TTI Bundling). In this technique, radio resources over a plurality of subframes (four TTIs, for example) are assigned to a particular user apparatus at one time, so as to improve signal quality from a user residing in the cell edge. After that, transmission and retransmission from the user apparatus are performed every multiple subframes as a whole.
FIG. 2 schematically shows a manner in which the subframe bundling is performed. Different from the case shown in FIG. 1, if retransmission is required after a signal is transmitted using a radio resource of 4 TTIs shown as A1 (occupying a bandwidth of one resource block in the frequency direction), the retransmission is also performed by using a radio resource (A2) of 4 TTIs. In the case of FIG. 2, since the user apparatus can transmit a signal using a radio resource four times greater than the radio resource in the case of FIG. 1, reception quality can be improved.
The subframe bundling is described in the non-patent documents 1 and 2, for example.